Press machines have been used to squeeze water out of a wet paper web in a paper manufacturing process. In a press machine, a wet paper web formed with layers therein is dewatered within a press nip, sandwiched between a pair of press felts. Press machines generally have a plurality of press nips.
FIG. 4 is a schematic view of a press nip in a press machine. A pair of press rolls P′, P′ and a pair of press felts 11′, 11′ form a press nip. The press felts 11′, 11′ and a wet paper web W′ are compressed within a pressure portion between the press rolls P′, P′, where water is removed from the wet paper web W′ and absorbed by the press felts 11′, 11′.
The volume of the wet paper web W′ and the press felts 11′, 11′ rapidly expands when they travel through the middle of the press portion (the nip) to the exit thereof, as they are rapidly released from compression. This expansion generates negative pressure within the press felts 11′, 11′ which, coupled with the capillary phenomenon within the wet paper web W′ associated with thin fibers therein, results in re-wetting, a phenomenon in which water absorbed by the press felts 11′, 11′ backs to the wet paper web.
Unexamined Japanese Patent Publication No. 143627/2004 discloses a press felt intended to prevent rewetting. This felt comprises a base layer, a wet paper web side batt layer, and press side batt layer, with a hydrophilic nonwoven fabric being disposed within the wet paper web side batt layer. According to this invention, the hydrophilic nonwoven fabric absorbs and retains water within the wet paper web side batt layer, thereby effectively preventing rewetting.
Moreover, it is also essential for a press felt to have a capability of recovering to its uncompressed state after compression without being flattened (resistance to compression fatigue), a capability of improving smoothness of the wet paper web by smoothness of the felt itself (smoothness), and dehairing and abrasion resistance.
Unexamined Japanese Patent Publication No. 302584/1996, for example, discloses a felt with such capabilities which includes fibers with a core-in-sheath structure made from a two-component material.
According to this invention, the two-component material used for a fiber to form a batt layer is composed of a sheath member with a low melting point and a core member with a high melting point. With heat hardening processing of the press felt, the sheath member with a low melting point gets softened to form a matrix within the batt layer, which enhances dewatering capability and compression resistance of the press felt.
Further, press felts made of a woven fabric with improved dewatering capability and smoothness are employed in recent high-speed paper manufacturing machines. The fabric is woven with a warp yarn (CMD yarn) and a weft yarn (MD yarn), both of which are monofilament single yarns (Unexamined Japanese Patent Publication No. 170086/2000).
Here, “machine direction (MD)” refers to the longitudinal direction in which a press felt is transferred in a paper manufacturing machine, whereas “cross machine direction (CMD)” refers to the lateral direction which crosses the machine direction.